Gel electrolyte capacitor

ABSTRACT

A poly(vinylphosphonic acid) (PVPA)-(NH 4 ) 2 MoO 4 ), gel polymer electrolyte can be prepared by incorporating redox-mediated Mo, or similar metal, into a PVPA, or similar polymer, matrix. Gel polymer electrolytes including PVPA/MoX, x representing the percent fraction Mo in PVPA, can be used to make supercapacitors including active carbon electrodes. The electrolytes can be in gel form, bendable and stretchable in a device. Devices including this gel electrolyte can have a specific capacitance (Cs) of 1276 F/g, i.e., a more than 50-fold increase relative to a PVPA system without Mo. A PVPA/Mo10 supercapacitor can have an energy density of 180.2 Wh/kg at power density of 500 W/kg, and devices with this hydrogel structure may maintain 85+% of their initial capacitance performance after 2300 charge-discharge cycles.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to electrolyte gels, particularlyflexible electrolyte gels, such as those comprising charged polymers,e.g., poly(vinylphosphonate), and a redox metal dopant, such as amolybdate, and to methods of making and using these, particularly incapacitors and other electronic devices.

Description of the Related Art

Interest toward flexible supercapacitors as energy storage devices hasincreased significantly in recent years because of the amendability offlexible supercapacitors to applications in wearable electronics, amongother things. Such electrochemical devices may have fastcharge-discharge rates, high power densities, low maintenance costs, andlong cycle lifetimes. The energy density of supercapacitors can stronglydepend on the composition of the electrolyte used.

Various aqueous electrolytes, such as H₂SO₄, KOH, or organic solvents(i.e., propylene carbonate), including salts, have been customarily usedin the art. Non-aqueous solvents also have been extensively used inelectric double layer capacitors (EDLCs), these electrolytes have somedisadvantages, including high flammability, toxicity, and liquidleakage, etc. One proposed avenue for preparing more stableelectrolytes, has involved replacing liquid electrolytes withelectrolytes having better dimensional stability.

Gel polymer electrolytes (GPEs) comprising a host polymer and one ormore dopants have been studied for increased dimensional stability. Gelelectrolytes can retain stability even at high temperatures, withoutusing an additional mechanical separator on a constructed device.However, the flammability of the GPE electrolytes is an important issuethat should be addressed, especially at higher temperatures or during adestructive accident. Additional properties, such as ionicconductivities and broad electrochemical stability windows, presentcritical problems to be solved.

Recently, capacitance of the supercapacitors has been improved by redoxreactions at the electrode-electrolyte interface upon addition of one ormore redox mediators into a polymer matrix. In a typical casecapacitance is increased by adding redox mediators, such as K₃Fe(CN)₆ orK₄Fe(CN)₆ in a KOH electrolyte, thereby leading to improved maximumcapacitance values of 712 F/g in the K₃Fe(CN)₆—KOH electrolyte and 317F/g in the K₄Fe(CN)₆—KOH electrolyte. Similarly, a capacitanceimprovement was observed for a carbon-based supercapacitor after addingKI to H₂SO₄, causing a capacitance enhancement from 472 F/g (without KI)to 912 F/g (with KI as mediator). The use of p-phenylenediamine as anorganic mediator in H₂SO₄ has been reported to improve the capacitanceof a supercapacitor from 144.1 F/g to 605.3 F/g. The specificcapacitance and energy density of a supercapacitor was reported toincrease by introducing Na₂MoO₄ into a polyvinyl alcohol (PVA)-H₂SO₄gel, forming Mo(VI)/Mo(IV) redox couples in the PVA-Na₂MoO₄—H₂SO₄ gelelectrolyte. Similarly, the capacitive effect of p-benzenediol in aPVA-H₂SO₄ electrolyte was reported to reach 474.29 F/g, far better thana PVA-H₂SO₄ system.

Research into capacitive performance of supercapacitors has not yet beenwidely reported for redox-mediated charged polymer-based gel polymerelectrolytes such as PVPA. Charged polymers, e.g., phosphate-basedelectrolytes, may offer physical and/or chemical solutions to some ofthe above-mentioned drawbacks of gel polymer electrolyte (GPE)supercapacitors.

Phosphorus-comprising polymers have been gaining interest in many fieldsdue to their flame resistance or inflammable character. This class ofpolymers may be functionalized either on the side chain or main chain,such as vinylphosphonates or polyphosphoesters. The phosphonate-linkedmonomer, vinylphosphonic acid (VPA), can yield poly(vinylphosphonicacid), PVPA, via polymerization. PVPA is an acidic polymer due to thephosphonic acid groups in its repeat unit, which may form an intrachainand interchain hydrogen bonding network. The conductivity of thesepolymer electrolytes may be increased by increasing of the concentrationof phosphonic acid groups, by doping, and/or the copolymerization withother comonomers. Homopolymers and copolymers of VPA have been used aspolymer electrolyte membranes, which are essential components of thefuel cell research.

US 2011/0272284 A1 by Elbick et al. (Elbick) discloses a process fortreating the surface of a Cr, Cu, Mn, Mo, Ag, Au, Pt, Pd, Rh, Pb, Sn,Ni, Zn, and/or Fe-comprising metal substrate. Elbick applies an anodicpotential to the metal surface in an electrolytic circuit of the metalsurface, a cathode, and an electrolytic solution in contact with themetal surface and the cathode. Elbick's electrolytic solution maycontain PO₄ ³⁻, RPO₃ ²⁻, HPO₃ ²⁻, R₂PO₂ ⁻, NO₃ ⁻, BO₃ ³⁻, SiO₄ ⁴⁻, MoO₄²⁻, WO₄ ²⁻, RCO₂ ⁻, and/or ⁻O₂CCO₂ ⁻ anion, which may comprise a polymerhaving a pendent phosphate, phosphonate, phosphite, phosphinate,sulfate, sulfonate, and/or carboxylate moiety. Elbick describespolyvinyl phosphonic acid and polyacrylic acid, but does not describeadding metallic redox ions to its electrolyte, e.g., (NH₄)₂MoO₄, nor anamount in the range of 1.0-20.0 wt. % of the total weight of the gelelectrolyte. Elbick's added salts, e.g., Na₃PO₄, Na₄SiO₄, H₃PO₄, Na₃PO₃,alkylphosphonates, alkylsulfates, etc., are for passivation ofsubstrates with a protective anodic layer.

U.S. Pat. No. 4,554,216 to Mohr (Mohr) discloses a process formanufacturing support materials for offset-printing plates in two stagesinvolving anodic oxidation in an aqueous electrolyte based on sulfuricacid, then in a different aqueous electrolyte. Mohr's differentelectrolyte has dissolved oxoanions of B, V, Mo, W, and/or C. Mohr doesnot disclose charged polymers, particularly poly(vinylphosphonic acid),nor a gel electrolyte.

CN 102421525 A by Chung et al. (Chung), which also published as US2012/0051999 A1 and US 2013/0004411 A1, discloses a catalyst comprisinga polyelectrolyte multilayer thin film, in which metal particles aredisposed on a carrier, to a method for producing same, and to a methodfor directly preparing hydrogen peroxide from oxygen and hydrogen usingthe catalyst. Chung's catalyst may use a cationic resin, anionic resin,and/or nonionic carrier, which may include poly(allylamine),polydiallyldimethylmonium, poly-(ethylenediamine),poly(acrylamide-co-diallyldimethylmonium), poly(4-styrenesulfonate),poly(acrylic acid), poly(acrylamide), poly(vinylphosphonic acid),poly(2-acrylamido-2-methyl-11-propanesulfonic acid), poly(anetholesulfonic acid), and/or poly(vinylsulfonate). Chung's dispersed metal maycomprise Pd, Pt, Ru, Rh, Ir, Ag, Os, Ni, Cu, Co, and/or Ti. Chung doesnot particularly describe redox metals, such as molybdates, normonolayers of electrolyte polymer.

U.S. Pat. No. 6,225,009 to Fleischer et al. (Fleischer) discloses anon-liquid electrolyte containing electrochemical cell which operatesefficiently at room temperature. Fleischer's cell includes a non-liquid,proton-mobile electrolyte, a proton-donating organic anode activematerial or a two-oxidation state metallic anode active material, and asolid couple-forming cathode. Fleischer's electrolyte may use H₂SO₄,CH₃SO₃H, HNO₃, HF, HCl, H₃PO₄, HBF₄, HClO₄, H₂SO₃, H₄P₂O₇, and/orpolyvinyl sulfonic and/or sulfuric acid. Fleischer's anodic activematerial may contain Sn, Ti, Cu, Al, W, Sb, Ir, Mo, Bi, and/or Cr.Fleischer's electrolyte may include various sulfonated polymers, waxes,or polyaromatics with a variety of vinyl polymers, including PVPA, butpreferably PVA. Fleischer's cathodic active material may includemolybdates, amount several. Fleischer's cells are generally asymmetricand Fleischer does not disclose a gel electrolyte of PVPA and ammoniummolybdate.

Japan. J. Appl. Phys. 2012, 51, 090121 by Kondo et al. (Kondo) disclosespolyoxometalates immobilized on a boron-doped diamond (BDD) surfacemodified by allyltriethylammonium bromide (ATAB), then immersed in aphosphomolybdic acid (H₃PMo₁₂O₄₀). Kondo also describespolyoxometalate-modified BDD from phosphonic-acid-terminated BDD,obtained by modifying BDD with vinylphosphonic acid (VPA), followed byreacting surface phosphonic acid groups with ammonium molybdate togenerate a lacunary phosphomolybdic acid (PMox) group. Kondo does notdisclose a polymerization of the VPA, nor an electrolyte comprisingPVPA, much less such an electrolyte comprising (NH₄)₂MoO₄ in a gel.

Int. J. Corros. Scale Inhib. 2014, 3(1), 28-34 by Neofotistou et al.(Neofotistou) discloses inhibiting silica polycondensations usingdendrimers based on polyaminoamide backbones with amine moieties assurface groups protonated to charge the dendrimer cationically.Neofotistou blends the cationic dendrimers with anionic polymers such aspolyvinylphosphonic acid for silica scale inhibition. Neofotistou doesnot disclose any gel electrolyte, nor capacitors, electrolytescomprising molybdates.

Appl. Organomet. Chem. 2011, 25(2), 128-132 and Inorg. Chem. Comm. 2011,14(3), 497-501 by Hu et al. (Hu) disclose heterogeneous catalysts forolefin epoxidation obtained by grafting diamines on organicpolymer-inorganic hybrid material, Zr poly(styrene-phenylvinyl-phosphonate)-phosphate (ZPS-PVPA), and subsequentlycoordinating with Schiff base Mo(VI) complexes. Hu does not pertain tocapacitors, nor does Hu use PVPA gel electrolytes comprising redoxmetals.

In light of the above, a need remains for electrolyte materials,particularly for capacitors and preferably flexible capacitors, whichmay take advantage of the properties of charged polymers, such as PVPA,and redox mediators, such as molybdates and similary situated materials,particularly for storing energy, as well as methods of making suchmaterials and capacitors.

SUMMARY OF THE INVENTION

Aspects of the invention provide electrolytes, which may comprise:poly(vinylphosphonic acid); and a redox mediator in an amount in a rangeof from 1.0 to 20.0 wt. % of a total electrolyte weight, wherein theelectrolyte is preferably in gel form. Such electrolytes can be modifiedby any permutation of the features described herein, particularly thefollowing.

The redox mediator may comprise a metal and/or a metalloid. The redoxmediator may comprise at least 75 wt. % of Mo, Cr, Ti, Zn, Ni, Rh, Ru,Os, Pd, Ce, W, Ta, Nb, V, Co, Mn, and/or Fe, relative to a totalelemental metal weight in the redox mediator, preferably Mo, such asmolybdate(s). The redox mediator may comprise (NH₄)₂MoO₄, e.g., in anamount of from 7.5 to 17.5 wt. % of the total electrolyte weight.

Aspects of the invention provide capacitors, which may comprise: a firstelectrically conducting layer; an electrolyte layer of any permutationof inventive electrolyte described herein; and a second electricallyconducting layer, wherein the electrolyte layer is sandwiched betweenthe layers of electrically conducting materials. Inventive capacitorsmay be symmetric. Such capacitors can be modified by any permutation ofthe features described herein, particularly the following.

The first and/or second electrically conductive layer may comprise atleast 50 wt. % activated carbon, relative to a total weight of theelectrically conductive layer. The first and/or second electricallyconductive layer may comprise conductive carbon in an amount of from 5to 25 wt. %, relative to a total weight of the electrically conductivelayer. The first and/or second electrically conductive layer maycomprise no more than 33 wt. % of a binder, relative to a total weightof the electrically conductive layer. The first and/or secondelectrically conductive layer may consist essentially of activatedcarbon, conductive carbon, and binder.

Inventive capacitors may maintain at least 85% of its specificcapacitance in a 60° bent and/or twisted state, relative to a flatstate. Inventive capacitors may comprise outer layers of aluminum,silver, gold, and/or copper. Inventive capacitors may have a specificcapacitance in a range of from 1000 to 1500 F/g, and/or an energydensity in a range of from 150 to 210 Wh/Kg at power density of 500W/kg.

Aspects of the invention provide methods of storing energy. Such methodsmay comprise: flowing current through a gel electrolyte layer comprisingpoly(vinylphosphonic acid) and a redox mediator in an amount in a rangeof from 1.0 to 20.0 wt. % of a total electrolyte layer weight.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 shows a representative synthetic scheme for the polymerization ofpoly(vinylphosphonic acid), PVPA;

FIG. 2 shows digital images of molybdenum-doped PVPA, redox-mediatedelectrolyte PVPA/MoX hydrogels;

FIG. 3 shows Fourier-transform infrared (FT-IR) spectra of exemplaryinventive PVPA/MoX materials with varied weight percent Mo-doping;

FIG. 4 shows thermogravimetric analysis (TGA) thermograms of dryexemplary inventive PVPA/MoX electrolytes with varied weight percentMo-doping;

FIG. 5 shows differential scanning calorimetry (DSC) plots of anexemplary inventive Mo-doped PVPA electrolytes with varied weightpercent Mo-doping;

FIG. 6 shows x-ray diffraction (XRD) patterns of exemplary inventivePVPA/Mo3 and PVPA/Mo10 materials;

FIG. 7 shows direct current (DC) conductvity of pure PVPA;

FIG. 8 shows cyclic voltammetry of pure PVPA-based supercapacitormeasured at different scan rates;

FIG. 9 shows charge-discharge curves of an exemplary inventivePVPA-based supercapacitor at a current density of 1 mA;

FIG. 10 shows comparative cyclic voltammetry (CV) curves of pure PVPA,pure ammonium molybdate (Mo), and an exemplary PVPA/Mo10 material-basedsupercapacitors;

FIG. 11 shows cyclic voltammetry curves of an exemplary inventivesupercapacitor including PVPA/Mo10 at different scan rates;

FIG. 12 shows galvanostatic charge-discharge (GCD) curves of anexemplary inventive PVPA/Mo₁₀ supercapacitor obtained at 1 mA currentdensity;

FIG. 13A shows electrochemical impedance spectroscopy (EIS) plots ofexemplary inventive PVPA/MoX hydrogels (X: 1, 5, 10, 20);

FIG. 13B shows the EIS plots of FIG. 13, expanded toward the origin;

FIG. 14 shows galvanostatic charge-discharge (GCD) curves of PVPA and anexemplary inventive PVPA/Mo10 material;

FIG. 15A shows galvanostatic charge-discharge (GCD) curves of anexemplary PVPA/Mo1 material under varied current;

FIG. 15B shows galvanostatic charge-discharge (GCD) curves of anexemplary PVPA/Mo5 material under varied current;

FIG. 15C shows galvanostatic charge-discharge (GCD) curves of anexemplary PVPA/Mo10 material under varied current;

FIG. 15D shows galvanostatic charge-discharge (GCD) curves of anexemplary PVPA/Mo20 material under varied current;

FIG. 16 shows galvanostatic charge-discharge (GCD) curves comparingexemplary PVPA/Mo1, PVPA/Mo5, PVPA/Mo10, and PVPA/Mo20 hydrogels at 1 mAcurrent density;

FIG. 17 shows specific capacitance variations of devices with selectedinventive PVPA/MoX samples with respect to current density;

FIG. 18 shows Ragone plots of the exemplary inventive supercapacitorsincluding PVPA/Mo1, PVPA/Mo5, PVPA/Mo10, and PVPA/Mo20 hydrogels;

FIG. 19 shows galvanostatic charge-discharge (GCD) curves obtained after10, 200, and 2500 cycles for an exemplary inventive PVPA/Mo10supercapacitor;

FIG. 20 shows plots indicative of the stability of the exemplaryinventive supercapacitors measured at a current density of 1 mA;

FIG. 21A shows a photographic image of an exemplary fabricated inventivesupercapacitor;

FIG. 21B shows a photographic image of an exemplary fabricated inventivesupercapacitor; and

FIG. 22 shows galvanostatic charge-discharge (GCD) curves of inventivesupercapacitors in free, bent, and twisted state.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Aspects of the invention provide electrolytes, which may comprise:poly(vinylphosphonic acid), for example, in an amount of at least 25,33, 40, 45, 50, 55, 60, 65, 70, or 75 wt. % and/or up to 50, 60, 70, 75,80, 85, 90, 95, 97.5, or 99 wt. %, of the total electrolyte weight; anda redox mediator in an amount in a range of from 1.0 to 20.0 wt. %,e.g., at least 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5,8, 8.5, 9, 9.5, or 10 wt. % and/or up to 20, 19.5, 19, 18.5, 18, 17.5,17, 16.5, 16, 15.5, 15, 14.5, 14, 13.5, 13, 12.5, 12, 11.5, 11, 10.5, or10 wt. %, of the total electrolyte weight, wherein the electrolyte ispreferably in gel form.

The term poly(vinylphosphonic acid), i.e., PVPA, may include analogs, ofPVPA, such as poly(vinylphosphonates) and/or poly(vinylphosphineoxides). Useful PVPAs may have a Brookfield viscosity (1% solution inH₂O) of at least 0.75, 0.8, 0.85, 0.9, 0.95, 1, 1.05, 1.1, 1.15, 1.2,1.25, 1.3, 1.35, 1.4, 1.45, or 1.5 cps and/or up to 1.5, 1.45, 1.4,1.35, 1.3, 1.25, 1.2, 1.15, 1.1, 1.05, or 1 cps. Useful PVPAs mayinclude copolymers of VPA with, e.g., acrylic acid, methacrylic acid,acrylonitrile, acrylamide, vinyl pyrrolidone, ethylene, vinylsulfonicacid, styrene, vinyl chloride, TFE, VDF, HFP, vinyl alcohol, vinylacetate, and/or propylene, e.g., as described in Macromolecules 2016,49, 2656-2662, Polym. Chem. 2013, 15(4), 4207-4218, Macromol. RapidComm. 2006, 27(20), 1719-1724, each of which is incorporated byreference herein in its entirety. Charged, particularly anionicallycharged, polymers may be used in place of or to supplement the PVPA. ThePVPA may be obtained indirectly, e.g., by polymerizing monomers likevinyl phosphonyl chloride or vinyl phosphonyl esters, which may besubsequently hydrolyzed or otherwise converted to PVPA. The PVPA,analog, or comonomer may be obtained by reversible additionfragmentation (RAFT), as described in the doctoral thesis entitled“Synthesis and Characterization of Poly(vinylphosphonic acid) for ProtonExchange Membranes in Fuel Cells” submitted by Bahar Bingoel at theJohannes Gutenberg-Universitat in Mainz in 2007, which is incorporatedby referenced herein in its entirety. Useful polymers may have amolecular weight (Mn) of, e.g., at least 5, 6, 7, 7.5, 8, 8.5, 9, 9.25,9.5, 9.75, 10, 10.5, 11, 12.5, 15, 20, or 25 kDa and/or up to 400, 375,350, 325, 300, 275, 250, 225, 200, 175, 150, 125, 100, 90, 80, 75, 70,65, 60, 55, 50, 45, 40, 35, 30, 27.5, 25, 22.5, 20, 17.5, or 15 kDa.Relevant polymers may have a polydispersity index (PDI) in a range of,e.g., at least 1.05, 1.1, 1.15, 1.2, 1.25, 1.33, 1.5, 2, 2.5, 3, 3.5, or4 and/or up to 10, 9, 8, 7.5, 7, 6.67, 6.33, 6, 5.75, 5.5, 5.25, 5,4.75, 4.5, 4.25, 4, 3.75, 3.67, 3.5, 3.33, 3.25, or 3.

The electrolyte will preferably behave as a gel, and may have propertieslike a Bingham fluid, a non-flowing non-solid, or an elastomer. Gel, asused herein, can mean viscoelastic materials generally which mayoptionally also lack thixotropy and/or thermoplasticity. The gel naturemay rely on the polyionic nature of the polymers used, such as PVPA,with ionic functional groups, such as —CO₂ ⁻, —SO₃ ⁻, and/or —PO₃ ²⁻.The ionic charges can prevent the formation of tightly coiled polymerchains, unlike customary uncharged polymers or lightly charged polymers.Such uncoiled nanomorphology can allows the polymers to contribute moreto viscosity in their stretched state, because the stretched-out polymertakes up more space. The charging on useful charged polymers within thescope of the invention may include at least 25, 33.3, 40, 50, 55, 65,75, 85, 80, 85, 90, 92.5, 95, 97.5, 98, 99, 99.1, 99.5, 99.9, 99.99 oreven 100% of the monomers containing a charged unit. Certainapplications may call for 1, 1.25, 1.5, 1.75, 2, 2.25, 2.5 or morecharges per monomer. Gels may be based on a polymer network formedthrough the physical aggregation of polymer chains, caused by one ormore of hydrogen bonds, crystallization, helix formation, complexation,etc., resulting in regions of local order acting as the network junctionpoints. Some such swollen networks may be thermoreversible gels if theregions of local order are thermally reversible. The gel may be ahydrogel, i.e., a gel in which the gelling agent is water. Useful gelsmay have mechanical properties as described in J. Appl. Polym. Sci.2001, 81(4), 948-956, J. Power Sources 2014, 245, 830-835, J. PowerSources 2018, 406, 128-140, Bull. Mater. Sci. 2003, 26(3), 321-328,and/or Solid State Ionics 1996, 85(1-4), 51-60, each of which isincorporated by reference herein it its entirety.

The redox mediator may comprise a metal and/or a metalloid. The redoxmediator may comprise at least 75, 80, 85, 90, 91, 92, 92.5, 93, 94, 95,96, 97, 97.5, 98, 99, 99.1, 99.5, or 99.9 wt. % of Mo, Cr, Ti, Zn, Ni,Rh, Ru, Os, Pd, Ce, W, Ta, Nb, V, Co, Mn, and/or Fe, relative to a totalelemental metal weight in the redox mediator, preferably Mo. Examples offorms such metals or metalloids may take could be oxides, salts, and/orcoordination complexes, for example, molybdate(s), tungstate(s),titanate(s), niobate(s), vanadate(s), manganate(s), permanganae(s),chromate(s), dichromate(s), selenate(s), cobaltate(s), titaniumoxide(s), zinc oxide(s), copper oxide(s), iron oxide(s), tin oxide(s),zirconium oxide(s), nickel oxide(s), osmium oxide(s), and/or ceriumoxide(s). Redox mediator complexes may comprise any of the relevantaforementioned metals and 1,10-phenanthroline (phen),4,4′-di-tertbutyl-2,2′-bipyridine (dtb), bipyridyl (bipy), hydrate, CO,CN, SCN, ammonia, chloride,2,2′-ethylenebis(nitrolomethylidene)diphenol-N,N′-ethylenebis(salicylimine)(salen), 2,4-di(pyrazol-1-yl)-1,3,5-triazine (bpt), quinquepyridine(qpy), 2,6-bis (1′-butylbenzimidazol-2′-yl) pyridine (dbbip),2,9-dimethy-1,10-phenanthroline (dmp), 3,4-ethylenedioxythiophene(EDOT), 4′-(3,4-ethylenedioxythiophene-2,2′:6′,2″-ter-pyridine (EtPy),4,4′,6,6′-tetramethyl-2,2′-bipyridine (tmby), 1-bis(2-pyridyl)ethane(bpye), [(−)-sparteine-N,N′]-(maleonitrile-dithiolato-S,S′) (SP)(mmt),acetylacetone (acac), 4,4-difluoro-1-phenylbutanate-1,3-dione (CF₂),dibenzoylmethanate (dbm), tetradentate diaminodiphenolate (hybeb),and/or terpyridine (tpy or terpy), etc., and mixtures of these,including mixed ligand complexes and multi-metal optionally mixed ligandcomplexes. Salts of relevant redox mediators may include ammonium,sodium, lithium, magnesium, potassium, and/or tetraalkylammonium (e.g.,(CH₃)₄N⁺, (CH₃CH₂)₄N⁺, ((CH₃)₂CH)₄N⁺, etc.). Exemplary redox mediatorsmay be, e.g., (NH₄)₂MoO₄, TiO₂, SrTiO₃, SnO₂, ZnO, WO₃, V₂O₅, CuO,Fe₂O₃, Os(bipy)₃, Ru(bipy)₃, (bipy)₂Ru(qpy)₃Ru(bipy)₂,(bipy)₂Os(qpy)₂Os(bipy)₂, (bipy)₂Ru(qpy)₂Ru(bipy)₂,(bipy)₂Os(qpy)₁Os(bipy)₂, (bipy)₂Ru(qpy)₁Ru(bipy)₂,(bipy)₂Ru(bpt)Ru(bipy)₂, (bipy)₂Os(bpt)Ru(bipy)₂,(bipy)₂Ru(bpt)Os(bipy)₂, (bipy)₂Os(pytr-bipy)Ru(bipy)₂,(bipy)₂Ru(pytr-bipy)Os(bipy)₂, (bipy)₂Ru(pytr-bipy)Ru(bipy)₂, Co(bpy)₃,Co(bpy)₃, Co(terpy)₃, Co(dbbip)₂, Co(phen)₃, Co(EtPy)₂, Co(dtb)₂,Cu(dmp)₂, Cu(SP)(mmt), Cu(phen)₂, Cu(bpy-(COOEt)₂)₂,Cu(bpy-(COOnbut)₂)₂, Cu(bpy-(COOtbut)₂)₂, Cu(bpye)₂, Cu(tmby)₂, Fe(CN)₆,Fe(phen)₃, Fe(bipy)₃, NiFe(CN)₆, Br₂Fc (ferrocene di-mono-bromide),BrFc, EtFc, Et₂Fc, Me₁₀Fc, Ni-bis(dicarbollide), Mn(acac)₃, Mn(CF₂)₃,Mn(dbm)₃, Mn(pzTp)₂, Mn(Tp)₂, Mn(Tp*)₂, VO(hybeb), and/or VO(salen). Theredox mediator may comprise (NH₄)₂MoO₄, and/or any other relevant redoxmediator, in an amount of from 7.5 to 17.5 wt. % of the totalelectrolyte weight, e.g., at least 5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5,10, 10.5, 11, 11.5, 12, or 12.5 wt. % and/or up to 18, 17.5, 17, 16.5,16, 15.5, 15, 14.5, 14, 13.5, 13, 12.5, 12, 11.5, 11, 10.5, or 10 wt. %.

Aspects of the invention provide capacitors, which may comprise: a firstelectrically conducting layer; an electrolyte layer of any permutationof inventive electrolyte described herein; and a second electricallyconducting layer, wherein the electrolyte layer is sandwiched betweenthe layers of electrically conducting materials. Inventive capacitorsmay be symmetric, i.e., having mirror image structure about a centralplane in the direction of the layering. Inventive capacitors maypreferably comprise only a single (gel) electrolyte layer, or 2, 3, 4,5, 6, 7, 8, 9, 10, or more layered (gel) electrolyte layers, alternatingwith (e.g., elemental carbon-based) electrically conducting layers. Thegel layer may be embedded in a mesh layer such that it is a compositegel layer or reinforced gel layer. That is, a support material such as amesh layer, screen, and/or film, including, for example aluminum, gold,copper, silver, and/or solid polymer, may support as a platform uponwhich the gel rests and/or as sandwiching surfaces on opposing sides ofthe gel. In certain applications, such mesh layers may be embedded inthe gel.

The first and/or second electrically conductive layer may comprise atleast 50, 60, 70, 75, 80, 85, 90, or 95 wt. % and/or up to 99, 97.5, 95,92.5, 90, 87.5, 85, 82.5, 80, 77.5, or 75 wt. % activated carbon,relative to a total weight of the electrically conductive layer. Theactivated carbon may comprise thermal black, furnace black, lamp black,and/or carbon aerogel. The activated carbon may be a powdered, granular,extruded, bead, impregnated, polymer-coated, and/or woven. The activatedcarbon may be carbonized under N₂ and/or Ar at a temperature of, e.g.,at least 500, 575, 600, 625, 650, 675, 700, 725, 750, 775, 800, 825,850, 875, or 900° C. and/or up to 1000, 975, 950, 925, 900, 875, 850,825, 800, 775, 750, 725, or 700° C., and/or activated/oxidized understeam, air, and/or O₂ at a temperature of, e.g., at least 850, 875, 900,925, 950, 975, 1000, 1025, 1050, 1075, 1100, 1125, 1150, 1175, and/or1200° C. and/or up to 1300, 1275, 1250, 1225, 1200, 1175, 1150, 1125,1100, 1075, 1050, 1025, 1000, 975, 950, 925, 900, 875, or 850° C.,and/or chemically activated, e.g., by phosphoric acid 25%, potassiumhydroxide 5%, sodium hydroxide 5%, calcium chloride 25%, or zincchloride 25%, at a temperature of, e.g., no more than 650, 625, 600,575, 550, 525, 500, 475, 450, 425, 400, 375, 350, 325, 300, 275, 250,225, or 200° C. The activated carbon may have a BET specific surfacearea of, e.g., at least 500, 550, 600, 650, 700, 750, 800, 850, 900,950, 1000, 1125, 1250, 1375, 1500, or 1750 m²/g and/or up to 3500, 3250,3000, 2900, 2800, 2750, 2700, 2600, 2500, 2400, 2300, 2250, 2200, 2150,2100, 2050, 2000, or 1750 m²/g. Relevant activated carbons may have aniodine number of, e.g., at least 450, 500, 550, 600, 650, 700, 750, 800,or 850 mg/g and/or up to 1250, 1200, 1175, 1150, 1125, 1100, 1075, 1050,1025, 1000, 975, 950, 925, 900, 875, 850, 825, 800, 775, or 750 mg/g.

The first and/or second electrically conductive layer may compriseconductive carbon in an amount of from 5 to 25 wt. %, relative to atotal weight of the electrically conductive layer, e.g., at least 5, 6,7, 7.5, 8, 8.5, 9, 9.5, 10, 11, or 12.5 wt. % and/or up to 25, 22.5, 20,17.5, 15, 14, 13, 12.5, 12, 11.5, 11, 10.5, 10, 9, or 8 wt. %. Theconductive carbon may comprise primary carbon, such as carbon black,which is generally amorphous and/or agglomerated, not graphite, coke, ordiamond. Useful conductive carbon may comprise carbon black having anaverage particle size of, e.g., at least 10, 12.5, 15, 17.5, 20, 22.5,25, 27.5, or 30 nm and/or 100, 95, 90, 85, 80, 75, 70, 65, 60, 55, or 50nm, and/or may have an average surface particle size of, e.g., 20, 25,50, 75, 100, 125, 150, 175, 200, 250, 300, 350, 400, 450, 500, 600, or750 m²/g and/or 1500, 1400, 1300, 1250, 1200, 1150, 1100, 1050, 1000,950, 900, 850, 800, or 750 m²/g.

The first and/or second electrically conductive layer may comprise nomore than 33, 30, 27.5, 25, 22.5, 20, 19, 18, 17.5, 17, 16, 15, 13.3,12.5, 12, 11, 10, 9, 8, 7.5, 7, 6, or 5 wt. % and/or at least 1, 2, 2.5,3, 4, 5, 7.5, or 10 wt. %, of a binder, relative to a total weight ofthe electrically conductive layer. Useful binders may include, forexample, poly(vinylidene difluoride) (PVdF), polytetrafluoroethylene(PTFE), styrene butadiene rubber (SBR), carboxymethyl cellulose (CMCoptionally as a salt, e.g., Na, K, Li, or the like), polyacrylic acid,polyethylene glycol (PEG), polyacrylonitrile, polystyene, polyurethane,polyisoprene, polyethylene, polypropylene, ethylene propylene dienemonomer (EPDM) rubber, poly(vinyl butyral), poly(vinyl acetate),poly(butyl acrylate), poly(methyl acrylate), chitosan, alginate,pectine, amylose, xanthan gum, gum arabic, gellan gum, Carrageenan,karaya gum, cellulose, guar gum, Tara gum, Tragacanth gum, gelatine,and/or caseinate.

The first and/or second electrically conductive layer may consistessentially of activated carbon, conductive carbon, and binder, in anypermutation described herein, i.e., have no less than 10, 7.5, or 5% ofthe specific capacitance in flat state without further components.Inventive capacitors may consist essentially of such first and/or secondelectrically conductive layers and a gel electrolyte in any permutationdescribed herein.

Inventive capacitors may maintain at least 85, 90, 91, 92, 92.5, 93, 94,95, 96, 97, 97.5, 98, 99, 99.1, 99.5, or 99.9% of its specificcapacitance in a 30, 45, 50, 55, 60, 65, 70, 75, 90° or more bent and/ortwisted state, relative to a flat state. Inventive capacitors maycomprise outer layers of aluminum, silver, gold, and/or copper.Inventive capacitors may have a specific capacitance in a range of from1000 to 1500 F/g, e.g., at least 900, 950, 1000, 1050, 1100, 1150, 1175,1200, 1225, 1250, 1275, 1300, or 1325 F/g and/or up to 1500, 1475, 1450,1425, 1400, 1375, 1350, 1325, 1300, 1275, 1250, 1225, or 1200 F/g.Inventive capacitors may have an energy density in a range of from 150to 210 Wh/kg, e.g., at least 150, 155, 160, 165, 167.5, 170, 172.5, 175,177.5, 180, 182.5, 185, 187.5, or 190 Wh/kg and/or up to 210, 205,202.5, 200, 197.5, 195, 192.5, 190, 187.5, 185, 182.5, or 180 Wh/kg, atpower density of 500 W/kg.

Aspects of the invention provide methods of storing energy. Such methodsmay comprise: flowing current through, or developing a charge separationacross, a gel electrolyte layer comprising a charged polymer, such aspoly(vinylphosphonic acid), and any redox mediator(s) described herein,preferably comprising a molybdate, in an amount in a range of from 1.0to 20.0 wt. % (or any percentage described herein) of a totalelectrolyte layer weight.

Inventive charged polymers may exclude acrylics, such as polyacrylate,polymethacrylate, etc., or may comprise no more than 15, 10, 7.5, 5, 4,3, 2, 1, or 0.5 wt. %, relative to the total electrolyte polymericweight, of any such acrylics, alone or in combination.

Inventive capacitors may exclude Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Zn,Cu, Sn, Zr, Cr, Pd, Pt, Ru, Rh, Ir, Ag, Os, Ni, Co, Ti, Sn, W, Sb, Mo,Bi, Cd, Pb, Se, Ta, V, Hf, Nb, and/Al, or may contain no more than 5, 4,3, 2.5, 2, 1, 0.5, 0.1, 0.01, 0.001, 0.0001, or 0.00001 wt. %, relativeto total elemental metallic weight, of any of these, individually or incombination. Inventive capacitors may exclude salts comprising sulfate,citrate, gluconate, nitrate, phosphate, phosphite, orthophosphate,silicate, selenate, tunstate, fluoride, chloride, bromide, iodide,carbonate, and/or bicarbonate, or may contain no more than 5, 4, 3, 2.5,2, 1, 0.5, 0.1, 0.01, 0.001, 0.0001, or 0.00001 wt. %, relative to totalelectrolyte weight, of any of these, individually or in combination.

Inventive electrolytes may exclude sugars, polyols, and/orpolysaccharides, e.g., maltose, digitonin, amygdalin, sucrose,pentaerythritol, glucose, cellobiose, mannose, inositol, starch,lactose, heparin, arabitol, dextrin, arabinose, erythritol, fructose,chitin, chitosan, gallactose, mannose, glucopyranose,tripentaerythritol, sorbitol, amylopectin, sorbitan (stearate),neuraminic acid, verbascose, threose, turanose, amylose, tagatose,trophanthobiose, sorbose, scillabiose, ribose, ribulose, rhamnose,raffinose, quinovose, quercitol, psicose, primeve rose, xylitol, xylose,naringin, mycosamine, muramic acid, methylglucoside, melezitose,melibiose, lyxose, lentinan, lactulose, inulin, hyalobiuranic acid,heptulose, guaran, glucosamine, gluconic acid, gluconolactone, gitonin,idose, fucose, and/or chondrosine, or may contain no more than 5, 4, 3,2.5, 2, 1, 0.5, 0.1, 0.01, 0.001, 0.0001, or 0.00001 wt. %, relative tototal electrolyte weight, of any of these, individually or incombination.

Inventive electrolytes may exclude sulfuric acid, phosphoric acid,molybdophosphoric acid, tungstophosphoric acid (TPA), sulfonated wax,polyvinylsulfonic acid, polyvinylphosphoric acid, sulfonatedpolyolefins, polyvinyl sulfuric acid, sulfonated polystyrene, sulfonatedphthalocyanine, sulfonated porphyrin,poly-2-acrylamido-2-methylpropanesulfonic acid, polyacrylic acid, and/orpolymethacrylic acid, or may comprise no more than 40, 33, 25, 20, 15,10, 7.5, 5, 4, 3, 2.5, 2, 1, 0.5, 0.1, 0.01, 0.001, 0.0001, or 0.00001wt. % of these individually or in combination, relative to the totalelectrolyte weight.

Inventive electrolytes may exclude polyethylene oxide, polyvinylacetate, polyacrylamide, polyethyleneimine, poly(vinyl pyrrolidone),poly(2-vinylpyridine), poly(4-vinylpyridine), polyvinylidene fluoride,polyhydroxyethylene, poly-2-ethyl-2-oxazoline, phenol formaldehyderesin, polyacrylamide, poly-N-substituted acrylamide,poly-N-vinylimidazole, agar, and/or agarose, or may comprise no morethan 40, 33, 25, 20, 15, 10, 7.5, 5, 4, 3, 2.5, 2, 1, 0.5, 0.1, 0.01,0.001, 0.0001, or 0.00001 wt. % of these individually or in combination,relative to the total electrolyte weight.

Inventive electrolytes may exclude quinone compounds, e.g., hydroquinonemonomethyl ether, hydroxy acetophenone, hydroxybenzaldehyde, hydroxybenzoic acid, hydroxybenzonitrile, acetaminophen, hydroxybenzyl alcohol,hydroxycinnamic acid, methylparabin, 2,5-dihydroxy-1,4-benzoquinone,resorcinol, ascorbic acid, ascorbic acid derivative, 1,4-dihydroxybenzene, 3-hydroxy tyramine (dopamine), rhodizonic acid, co-enzyme Q,1,2,3-trihydroxy benzene (pyrogallol), 1,3,5-trihydroxy benzene(phloroglucinol), tetrahydroxy quinone (THQ), tetrahydroxy acetophenone,tetrahydroxy benzoic acid, hexahydroxy benzene, tetrahydroxy quinone,hexahydroxybenzene, chloranilic acid, chloranilic acid, chloranil,rhodizonic acid, fluoroanilic acid, reduced fluoroanilic acid,fluoranil, duroquinone, 1-nitroso-2-napthol, martius yellow,hydroxy-1,4-naphthaquinone, naphthalene diol, tetrahydroxy napthalene,tetrahydroxy 1,4-naphthaquinone, echinochrome, pentahydroxy1,4-naphthaquinone, anthranol, hydroxy anthraquinone, anthralin,anthrarufin, alizarin, di-hydroxyanthraquinone, anthrobin, anthragallol,purpurin, 1,8,9-anthracenetriol, 1,2,5,8-tetrahydroxyanthraquinone,carminic acid, purpogallin, hydroxybenzophenone, hydroquinonemonobenzylether, hydroxy biphenyl, 2,2,4,4-tetrahydroxy benzophenone,phenolphthalein, indophenol, bromophenol blue, methylenedigallic acid,methylenedisalicyclic acid, 5-hydroxy-2(5H)-furanone, hydroxycourmarin,fustin, hydroxindole, tetrahydropapaveroline, oxindole,o-phenanthroline, phenanthridine, 6(5H)phenanthridinone,hydroxyjulolidine, citrazinic acid, uracil, 2-amino-5-bromopyridine,5-aminotetrazole monohydrate, 2-aminothiazole, 2-aminopyrimidine,2-amino-3-hydroxypyridine, 2,4,6-triaminopyrimidine,2,4-diamino-6-hydroxy pyrimidine, 5,6-diamino-1,3-dimethyluracilhydrate, 5,6-diamino-2-thiouracil, cyanuric acid, and/or hydroxy methylpyridine, or may comprise no more than 5, 4, 3, 2.5, 2, 1, 0.5, 0.1,0.01, 0.001, 0.0001, or 0.00001 wt. % of these individually or incombination, relative to the total electrolyte weight.

Aspects of the invention introduce molybdate salts, such as ammoniummolybdate, into PVPA and/or use Mo as redox mediator in PVPA at variousconcentrations to obtain hydrogels, PVPA/MoX, e.g., 1, 2, 2.5, 3, 4, 5,6, 7, 7.5, 8, 9, or 10 wt. % and/or up to 30, 25, 22.5, 20, 19, 18,17.5, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, or 7.5 wt. %.Supercapacitors including such PVPA/MoX materials may also use activatecarbon (AC) electodes.

Aspects of the invention may comprise rational designs ofsupercapacitors, particularly comprising redox-mediated electrolyte, forexample, PVPA/MoX and optionally using an active carbon electrode.Inventive hydrogels can be prepared with different weight percentages ofMo in PVPA or similar polymer matrices, e.g. ranging from at least 1, 2,3, 4, 5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12, or 12.5wt. % and/or up to 50, 45, 40, 35, 33.3, 30, 27.5, 25, 22.5, 20, 19, 18,17.5, 17, 16.5, 16, 15.5, 15, 14.5, 14, 13.5, 13, 12.5, 12, 11.5, 11,10.5, or 10 wt. %. Aspects of the invention may include enhancingpseudocapacitance and/or sustaining electrical double layer capacitanceby doping charged polymers with particular metal ions and/or oxides, forexample, of Mo, W, Cr, V, Nb, Ta, Mn, Co, etc., such as molybdate,tunstate, vanadate, cobaltate, (per)manganate, and the like.

Aspects of the invention may provide improved electric double layercapacitance and/or pseudocapacitance, e.g., to increase the dischargetime at least 25, 30, 35, 40, 42.5, 45, 47.5, 50, 52.5, 55, 57.5, 60,62.5, 65, 67.5, 70, 75, 80, 85, 100-fold or more relative to the purecharged polymer matrix and/or polyelectrode, such aspoly(vinylphosphonic acid), PVPA. Inventive capacitors may sustain acapacitance of at least 1276 F/g, e.g., at least 1000, 1025, 1050, 1075,1100, 1125, 1150, 1175, 1200, 1225, 1250, 1275, 1300, 1325, 1350, 1375,or 1400 F/g, and/or up to 2000, 1950, 1900, 1850, 1800, 1750, 1700,1650, 1600, 1550, or 1500 F/g. Inventive capacitors may limit loss to nomore than 20, 19, 18, 17.5, 17, 16, 15, 14, 13, 12.5, 12, 11, 10% orless loss, e.g., after at least 1750, 2000, 2100, 2150, 2200, 2250,2300, 2350, 2400, 2500, 2750, 3000, or 3500 cycles and/or up to 10000,7500, 7000, 6500, 6000, 5500, 5000, 4750, 4500, 4250, 4000, 3750, or3500 cycles.

Inventive supercapacitors may employ PVPA with, e.g., 10±0.1, 0.5, 0.75,1, 1.5, 2, 2.5, 3, 4, 5, 6, or 7.5 wt. % Mo. Inventive supercapacitorsmay have an energy density around 180.2 Wh/kg, e.g., at least 160,162.5, 165, 167.5, 170, 172.5, 175, 177.5, 178, 179, 180, 181, 182,182.5, 183, 184, or 185 Wh/kg and/or 200, 197.5, 195, 192.5, 190, 187.5,185, 184, 183, 182.5, 182, 181, 180, 179, 178, 177.5, 177, 176, or 175Wh/kg, at power density of 500±2.5, 5, 10, 15, 25, 35, 50, 75, 100, or150 W/kg. Inventive supercapacitors may be flexible, and/or may besuitable for twisting, e.g., at least 1, 2, 3, 6, 9, 12, 15, 18, 21, 24,27, 30, 45, or 60° and/or up to 180, 150, 120, 90, 75, 60, 45, 42, 39,36, 33, or 30° torsionally about a hypothetical central axis, and bentstates, e.g., at least 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 45, 60, 75,or 90° and/or up to 180, 150, 120, 90, 60, 45, 42, 39, 36, 33, or 30°.Aspects of the invention may provide supercapacitors comprising suchredox doped, charged polymers.

EXAMPLES

Materials: Alpha,alpha′-azodiisobutyramidine dihydrochloride(AIBHC, >98% Fluka), vinylphosphonic acid (>97%, Aldrich).Polyvinylidene fluoride (HSV 900 PVDF) (MTI), activated carbon (AC), andconductive carbon (CC) (MTI), conductive additive (Timical super C65application) (MTI). Ethanol and 1-methyl-2-pyrrolidone (NMP) werereceived from Merck.

Electrode and Electrolyte Preparation: Poly(vinylphosphonic acid) wassynthesized as described in J. Non-Cryst. Solids 2008, 354(30),3637-3642, which is incorporated by reference herein in its entirety,whereby the monomer, vinyl phosphonic acid (VPA) was free radicalpolymerized to produce PVPA. The polymerization was carried out using aninitiator, azodiisobutyramidine dihydrochloride (0.1 mol. %), at 70° C.for 3 hours. After the polymerization, the homopolymer was purified.Polymerizations can be carried out by any manner known in the art.

Redox mediator, ammonium molybdate was introduced into thepolyelectrolyte, PVPA, at various weight fractions in aqueous solution.The hydrogels were abbreviated as PVPA/MoX, X being the weightpercentage of Mo in the PVPA, with samples ranging from 1 to 20%.

Fabrication of Supercapacitor Electrodes: The supercapacitor electrodeswere made containing activated carbon (CA—Kuraray active carbon forsupercapacitor electrode MTI), conductive carbon (CC—Timical super 65,conductive additive for Lithium ion batteries), and binder PVDF atvarious contents. A slurry including 80 wt. % CA, 10 wt. % CC, and 10wt. % PVDF were prepared by mixing at 70° C. After homogenization, themixture was cast onto an aluminum mesh foil using an MRX automaticcoating machine (Shenzhen Automation Equipment). Finally, the electrodewas dried in an oven at 80° C.

Fabrication of Flexible Supercapacitor Devices: Supercapacitor deviceswere assembled with a structure: Al/AC+CC/PVPA/MoX/CC+AC/Al, i.e.,aluminum, activated carbon/conductive carbon/binder, Mo-doped PVPA,conductive carbon/activated carbon/binder, aluminum. PVPA/MoX hydrogelswere cast onto the surface of AC+CC electrodes. Supercapacitor cellswere placed in SWAGELOK® cell kit for electrochemical testing.

Cyclic voltammetry (CV) studies was performed by using Palmsens EmStat³electrochemical analyzer. The CV traces of supercapacitor cells wereevaluated in the potential range of 0.0 to 1 V at different scan rates,ranging from 10 to 400 mV/s. Galvanostatic charge-discharge (GCD)experiments were carried out by MTI battery analyzer at the currentdensities 1 to 10 A/g and cut off voltage 0.1 to 1 V.

The device specific capacitances (C_(s)) of the symmetricalsupercapacitors were calculated based on the Equation 1, below, withcurrent density increasing from 1 to 10 A/g:

C _(s)=2·I·Δt/m·ΔV  Eq. 1,

wherein ΔV is voltage difference in discharge, I is discharge current,Δt is discharge time, and m is the mass of the electrode activematerial.

Energy and power densities of symmetric supercapacitors were calculatedbased on Equations 2 and 3, below:

E=(½·C _(s)·(ΔV ²)]/3.6  Eq. 2, and

P=E·(3600/Δt)  Eq. 3,

wherein E is energy density, P is power density, ΔV is voltage window,and Δt is discharge time.

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views.

FIG. 1 shows a representation of the preparation of the polyelectrolyte,poly(vinylphosphonic acid) (PVPA), polymerized from vinylphosphonic acid(VPA) monomer, whereafter the dopant, e.g., a Mo compound (or anydescribed above), can be incorporated into the matrix, e.g., PVPA (orany described above), in a desired weight percentage to obtain hydrogelsincluding redox active metal ions. It is not precluded that the dopantbe included in the monomer mixture insofar as this does not inhibit thepolymerization.

FIG. 2 shows digital photographs of the exemplary PVPA/MoX hydrogels. Bychanging the concentration of the Mo ions in the matrix, no color changewas noticed for PVPA based electrolyte.

FIG. 3 shows the Fourier-transform infrared (FT-IR) spectrum ofexemplary inventive PVPA/MoX based electrolyte systems. The FT-IRspectrum of ammonium molybdate comprises several strong absorption peaksat 900, 815, 622, and 550 cm⁻¹, belonging to stretching and bendingvibrations of Mo—O and Mo—O—Mo. The characteristic broad peak between990-910 cm⁻¹ can be attributed to (P—O)—H stretching, and the peak at1150 cm⁻¹ belongs to P—O of phosphoric acid groups of the PVPA. Thephosphonic acid group gives additional broad band in the region of 1635cm⁻¹. The broadening between 3300 and 2000 cm⁻¹ can be attributed tohydrogen boding network formation among phosphonic acid groups. Theintroduction of Mo results in a new peak at 716 cm⁻¹ due to Mo—O—Mostretching and this peak became more pronounced at higher doping ratios.

FIG. 4 shows a thermogravimetric analysis (TGA) curve of exemplaryinventive PVPA/MoX electrolytes, demonstrating weight changes in roughlythree steps or stages. The first stage starts at roughly 155° C. andends at roughly 262° C., constituting a 10% weight loss that can beattributed to condensation of phosphonic acid groups. The second stagebegins above 270° C. and may be due to the loss of ammonia and furthercondensation of acidic units. The third stage may correspond to thedegradation of the redox mediated electrolytes, i.e., PVPA/Mo1,PVPA/Mo5, PVPA/Mo10, and PVPA/Mo20, respectively starting at 437, 439,439, and 442° C. The increase in the Mo fraction results in improvedthermal stability of the PVPA/MoX electrolytes.

FIG. 5 shows differential scanning calorimetry (DSC) plots to study theglass transition temperatures (T_(g)) of the dried inventive PVPA/MoXelectrolytes. The T_(g) of the PVPA/Mo1 sample was measured to be 38.5°C., and the T_(g) of the PVPA/Mo3 sample was measured to be 32.6° C. Asevidenced by the second plot from the top, increasing the content of Mofrom 5 to 10 wt. % shifts the T_(g) to 47° C. for the PVPA/Mo10 sample,while no Tg was observed for the PVPA/Mo20 sample. The behavior of thePVPA/Mo20 sample may be described by the complexation of the redoxadditive by the polymer, thereby reducing the cooperative segmentalmotions of the polymer chains.

FIG. 6 shows the x-ray diffraction (XRD) patterns of dried inventivePVPA/MoX samples. The patterns illustrate typical amorphous nature inboth PVPA/Mo3 and PVPA/Mo10 electrolytes. Further broadening andshifting of the peak from 23.2 to 22.6° (20) occurred with increased Mocontent. This broadening behavior may indicate complexation between Moand PVPA to increase the amorphous character of the final material.

FIG. 7 shows the direct current (DC) conductivity of the PVPA derivedfrom alternating current (AC) conductivity. The ion conductivity of thepure polymer electrolyte depends on the temperature, and ranges from2.3×10⁻⁵ to 4.1×10⁻³ S/cm.

FIG. 8 shows cyclic voltammetry traces performed in the presence of neatpolyelectrolyte (PVPA) at different at scan rates from 10 to 250 mV/swithin potential window of 0 to 1 V. The voltammogram of a symmetricaldevice with a configuration electrode/PVPA/electrode shows aquasi-rectangular shape, indicating the formation of anelectric-electrochemical double-layer capacitor (EDLC). The rate ofcurrent density increases with increasing scan rate, indicating fast iontransfer capability of the PVPA polyelectrolyte. The acidic groups inthe polyelectrolyte may provide the ion dissociation and diffusionbetween the carbon electrodes on current collectors.

FIG. 9 shows typical galvanostatic charge-discharge (GCD) curves of asupercapacitor including as a hydrogel. The supercapacitor was tested byscanning a charging voltage to 1 V and discharging voltage to 0 V with acurrent density of 1 mA. The GCD curves indicate that the system hasfast voltage drops during the discharge time. Such fast voltage dropsdemonstrate that high capacitance values cannot be achieved as such inelectrochemical supercapacitors using pure PVPA polyelectrolyte.

FIG. 10 shows cyclic voltammetry (CV) measurements of exemplaryinventive PVPA hydrogels comprising Mo performed in a solutioncontaining 0.1 M KCl and 0.01 M HCl. The voltammogram of a purePVPA-based supercapacitor with activated carbon (AC) electrodes at ascan rate of 100 mV/s is illustrated in the center right of the chart.Due to the stable structure of the PVPA hydrogel, no oxidation-reductionpeak was observed within the scanned potential range for pure PVPA. TheCV curve of the pure Mo shown at a potential window of −1 V to +1 V anda scan rate of 100 mV/s is the largest trace. The Mo trace shows twostrong redox peaks of Mo on the potential window under the samecondition, in the range of −0.1 V/0.4 V for oxidation-reduction iontransitions of Mo (VI)/Mo (IV) and in the range of 0.5 V/0.1 V foroxidation-reduction ion transitions of Mo (VI)/Mo (V). These redoxbalances are represented by Equations 4 and 5, below.

H₂Mo(IV)O₃+H₂O→H₂Mo(VI)O₄+2e ⁻+2H⁺  Eq. 4

HMo(V)O₃+H₂O→H₂Mo(VI)O₄ +e ⁻+H⁺  Eq. 5

MoO₄ species have been reported to tend to pass to the polymeric ionstructure (H₂MoO₄) at oxidation level of +VI in acidic medium. Possiblydue to the acidic nature of the PVPA hydrogel, the transformation ofmolybdate into H₂MoO₄ was noticeable in the CV diagram.

As seen in FIG. 10, the ion transfer throughout the polymeric hydrogelincreased by increasing the scan rate from 20 to 250 mV/s. The intensityof reversible redox peaks are also increased by increasing the scan ratefrom 20 to 250 mV/s, which indicates charge transfer capability withinthe exemplary inventive PVPA/Mo hydrogels. In addition, the CV peaks ofthe Mo-containing hydrogel maintained its behavior at low and highscanning rates, demonstrating electrochemical stability as well asexcellent charging performance of the inventive PVPA/Mo hydrogel.

FIG. 11 shows the galvanostatic charge-discharge (GCD) curves of theexemplary inventive Mo-doped PVPA hydrogels, carried out as the systemcharging to 1 V and discharging to −1 V with a current density of 1 mA.Excellent performance enhancement was observed after insertion of Mointo the hydrogels. The GCD curves of the exemplary PVPA/Mo hydrogelsshowed ideal electrochemical supercapacitor behavior with extendedcharging and discharging time, particularly relative to the PVPA-basedsupercapacitor. These results indicate that capacitance values can beincreased, e.g., at least 40, 42.5, 45, 47.5, 50, 52.5, 55, 57.5, 60,65, 75, 85-fold or more, by doping hydrogels with Mo and/or similarredox metals, indicating outstanding charge storing capability.

FIGS. 13A and 13B show Nyquist plots for different concentrations ofMo-doped PVPA-based supercapacitors, i.e., PVPA/MoX devices. Theinventive device electrodes were found to have resistances of 0.33 ohmfor PVPA/Mo1, 0.55 ohm for PVPA/Mo5, 0.92 ohm for PVPA/Mo10, and 2.21ohm for PVPA/Mo20. The intersection of the Nyquist plots at the x-axisequals the equivalent series resistance (ESR). The Warburg resistance(W) of the exemplary inventive systems at low frequency area indicatesthe diffusion of ions through the pores in the surface of theelectrodes. The Warburg impedance of all inventive supercapacitorsamples indicated that the activated carbon and PVPA/Mo system allow iondiffusion on the electrode surface, which can increase thecharge-discharge performance of the system. The charge transferresistance (R_(ct)) values of the symmetrical supercapacitors obtainedfrom the diameter of the semicircle at high frequency region were 2.27ohm for PVPA/Mo1, 3.85 ohm for PVPA/Mo5, 5.57 ohm for PVPA/Mo10, and9.85 ohm for PVPA/Mo20.

FIG. 14 shows comparative galvanostatic charge-discharge (GCD) curvesobtained from supercapacitors including PVPA and PVPA/Mo10 hydrogels.The GCD traces of symmetrical PVPA based supercapacitors were performedby applying the potential from 0 V to 1 V at current density of 1 mA.After addition of redox active Mo into the PVPA, the voltage window wasextended within the electroactive region of the Mo. The GCD measurementswere carried out between −1 V to 1 V, based on the redox peaks observedin the cyclic voltammograms. Supercapacitor containing Mo-dopedelectrolyte revealed more than 50 times higher discharge time comparedto the pure PVPA in FIG. 14. The electrochemical oxidation-reductionreactions of Mo ions are observed to increase the charge storageperformance of the supercapacitor.

FIG. 15A to 15D show charge-discharge measurements of the exemplaryinventive supercapacitors comprising PVPA/Mo hydrogels at differentcurrent densities. The galvanostatic charge-discharge (GCD) curves ofdifferent supercapacitors comprising PVPA/MoX (X=1, 5, 10, and 20)electrolytes at current densities of 1 mA to 10 mA. Supercapacitorscontaining Mo show pseud-ocapacitive GCD trends and different and highercapacitance values compared to systems with only PVPA. Such behavior wassupported by cyclic voltammograms and can be attributed to the redoxbehavior of Mo ions on the electrode surface. The increase in thecurrent density can be seen to diminish the charge and discharge timesof the device, demonstrating higher charge transfer over time. The plotsshow that all the discharge curves have a rapid voltage loss, and thisdrop is further increased at higher current densities, i.e., above 4,4.5, 5, 6, or 7.5 mA. Despite the rapid voltage loss, a limitedreduction in capacitance values occurred. This may be explained by ahigh ion transfer capability of inventive PVPA/MoX hydrogels allowingthe charge storage at any potential level.

FIG. 16 shows the comparison of galvanostatic charge-discharge (GCD)curves of PVPA hydrogel-based supercapacitors with different Mo dopingfractions at a current density of 1 mA. The exemplary inventivePVPA/Mo10-based supercapacitor provides the highest capacitance, as wellas the maximum discharge time, under identical conditions.

FIG. 17 shows the specific capacitances (C_(e)) of pure PVPA andexemplary inventive PVPA/MoX-based symmetrical supercapacitors, obtainedat potentials ranging from 0.0 to 1 V and −1 to 1 V and varied currentdensities, i.e., 1, 2, 3, 5 and 10 mA. The supercapacitor comprisingPVPA shows a C_(s) of 50 F/g at a current density of 1 A/g. Animprovement was achieved after insertion of Mo into the hydrogel and itincreased up to 1276 F/g in the sample containing PVPA/Mo10 hydrogel.The C_(s) of the supercacitor with PVPA/MoX increased with x up to 10%,reaching the highest performance tested for PVPA/Mo10. Increasing theconcentration of Mo ions from 10 to 20 wt. % resulted in a decrease inthe C_(s) at the same current density. The highest C_(s) values wereobserved at a low current density, which may be due to the motion ofions between electrodes having the maximum possible ion diffusion to bestored by all layers of the active material. At high current density,the effective use of the electrode active material is limited only tothe outer surface of the active material, which can reduce the specificcapacitance value. Following this theory, redox active Mo ions can reachall layers of active electrode material and increase the charge storagecapability by their redox active functionality.

FIG. 18 shows plots of the power density and energy density of PVPA andexemplary inventive PVPA/MoX hydrogel-based symmetric supercapacitors.The plots show that the supercapacitor comprising PVPA/Mo10 has thehighest energy density tested, i.e., 180.2 Wh/kg at power density of 500W/kg, and an energy density of 170.2 Wh/kg at a power density of 1.000kW/kg. Still higher energy of 148.2 Wh/kg could be harvested at a powerdensity of 5200 W/kg. This behavior may be explained by supercapacitorswith PVPA/Mo hydrogel losing their capability faster at high rates,leading to more power loss.

FIG. 19 shows test results for the specific capacity of the exemplarysupercapacitor comprising the PVPA/Mo10 hydrogel, comparing thegalvanostatic charge-discharge (GCD) curves versus cycle count. Anunexpectedly superior performance up to 100th cycle (FIG. 19 showscomparative Cp at initial 10 cycle, 200 cycle and 2500 cycles) wasobserved for the device comprising the PVPA/Mo10 hydrogel, which hasspecific capacitance of 1250 F/g. After that, a capacitance loss wasobserved up to 200th cycle (middle curve), indicating that the devicereached stabilization. This can be attributed to the fast diffusion ofMo ions, yielding high rates in the first 200 cycles. After the 200thcycle, the electrochemical redox reactions may be reduced by diminishingnumbers of ions slightly decreasing the performance of the device.Comparing the GCD curves after the 200th and 2500th cycles, the devicecomprising the PVPA/Mo₁₀ hydrogen lost ca. 15% of its initialperformance after 2300 GCD cycles.

FIG. 20 shows experiments on the cyclability of the differentPVPA/Mo_based supercapacitor devices performed by applying 2500galvanostatic charge-discharge (GCD) cycles. Tests were performed usingsimilar supercapacitor devices at an applied current density of 1 mA.The PVPA/Mo10-containing supercapacitor showed comparable performanceafter 2500 cycles. All devices tend to lose a performance in the first200 cycles, which may be due to the electrode polyelectrolyte reachingthe maximum ion transfer ability. The PVPA/Mo10 device retained thealmost 85% of its performance between 200 and 2500 cycles.

FIGS. 21A and 21B show digital photographs of a device in twisted andbent state. The performance of the supercapacitor including PVPA/Mo₁₀hydrogel under flexible conditions was also carried out as shown in FIG.22. Galvanostatic charge-discharge (GCD) measurements were performedwith a device in different states of contortion: free-standing, bentstate at 60° angle; and twisted state. No significant change in specificcapacitances for all different states, i.e., 987 F/g free standing, 980F/g at 60° bent, and 978.5 F/g in twisted state, was observed at acurrent density of 1 mA. This result indicates that the device hasexcellent capacitance stabilization due to the flexibility, and that itscapacitance is diminished no more than 2.5, 5, 7.5, 10, or 12.5% invarious bent and/or twisted states, e.g., 30, 45, 60, 75, or 90°.

Numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

1-7. (canceled)
 8. A capacitor, comprising: a first electricallyconducting layer; an electrolyte layer comprising: poly(vinylphosphonicacid); and a molybdate redox mediator in an amount in a range of from1.0 to 20.0 wt. % of a total electrolyte weight, wherein the electrolyteis in gel form; and a second electrically conducting layer, wherein theelectrolyte layer comprises a mesh reinforcement and is sandwichedbetween the layers of electrically conducting materials.
 9. Thecapacitor of claim 8, which is a symmetric capacitor.
 10. The capacitorof claim 8, wherein the first and/or second electrically conductivelayer comprises at least 50 wt. % activated carbon, relative to a totalweight of the electrically conductive layer.
 11. The capacitor of claim8, wherein the first and/or second electrically conductive layercomprises conductive carbon in an amount of from 5 to 25 wt. %, relativeto a total weight of the electrically conductive layer.
 12. Thecapacitor of claim 8, wherein the first and/or second electricallyconductive layer comprises no more than 33 wt. % of a binder, relativeto a total weight of the electrically conductive layer.
 13. Thecapacitor of claim 8, wherein the first and/or second electricallyconductive layer consist essentially of activated carbon, conductivecarbon, and binder.
 14. The capacitor of claim 8, wherein the redoxmediator comprises a metal.
 15. The capacitor of claim 8, wherein theredox mediator comprises at least 75 wt. % Mo, relative to a totalelemental metal weight in the redox mediator.
 16. The capacitor of claim8, wherein the redox mediator comprises a molybdate (NH₄)₂MoO₄ is in anamount of from 7.5 to 17.5 wt. % of the total electrolyte weight. 17.The capacitor of claim 8, which maintains at least 85% of its specificcapacitance in a 60° bent and/or twisted state, relative to a flatstate.
 18. The capacitor of claim 8, comprising outer layers ofaluminum, silver, gold, and/or copper.
 19. The capacitor of claim 8,having a specific capacitance in a range of from 1000 to 1500 F/g,and/or an energy density in a range of from 150 to 210 Wh/kg at powerdensity of 500 W/kg.
 20. (canceled)